An unjust tax?

From Drudge:

TACO BELL TAX: New Jersey lawmakers considering 'sin' tax on fast food...

If it is a sin tax, would it not presume that people are able to avoid the use of the product? But is this the case with the poor and the lower middle class? What if a family decides that it is more time-saving to eat at a McDonalds than to buy and cook, and thus better for the family? Might there not be other reason that would justify frequent visits to a fast-food restaurant? I tend to agree that this tax would be too much of a burden on certain groups, especially when lawmakers fail to protect their freedom to be free from relying on fast food.